Where bricks are made on a large scale the work is now done almost entirely by machinery, commencing with the mining of the clay by steam shovels and ending by burning in patent kilns.

A great variety of machines are now made for preparing the clay and for making the raw bricks; they differ more or less widely in construction and principle, but may be divided into three classes, according to the method of manufacture for which they are adapted-There are practically three methods employed in making bricks, viz.: The soft mud process, the stiff mud process and the dry clay process, and the machines are also classed under one of these headings.

The general processes employed in these methods are as follows : Soft Mud Process. - This is essentially the same process as that employed when the bricks are made by hand. When machinery is used the various steps are about as follows : As the clay is brought from the bank it is thrown into a pit (about 6 feet deep and 8x12. feet in area) lined with planks; water is then turned into the pit and the clay allowed to soak for twenty-four hours. Generally three pits are provided, so that the clay in one may be soaking while the second is being emptied and the third filled. If coal dust is to be mixed with the clay it is thrown into the pit in the proper proportion. After-soaking twenty-four hours in the pit the clay is thrown out on to an endless chain, which carries it along to the machine, into which it falls. The upper part of a soft clay machine contains a revolving shaft, to which arms are affixed. These arms break up and thoroughly work the soft clay, and it falls to the bottom of the machine, where revolving blades force it forward, and a plunger working up and down forces the clay into a mould placed under the orifice. The filled mould is then drawn or forced out on to a shelf or table and another mould placed under the machine. There are several styles of machines, but they all work on about this plan. Sometimes the clay is worked in a pug mill before being thrown into the machine. After being drawn from the machine the filled moulds are emptied by hand and the bricks taken to the dry shed. For drying soft mud bricks the "pallet" system is generally employed. The "pallets" are thin boards about 12x24 inches in size. The bricks are placed on these, and then the pallets are placed on racks, arranged so that the air may have free access to the bricks. The stacks should always be protected by a low roof.

2l8. Stiff Mud Process. - The essential difference between this process and the foregoing is that in the stiff mud process the clay is first ground, or disintegrated, and only enough water is added to make a stiff mud. The mud, after being pugged, is forced through a die in a continuous stream, whose section is the size of a brick, and the bricks are then cut off.

The process varies more or less in different yards and with different clays, but when most thoroughly carried out the various steps in their order are as follows: First, the mining of the clay; second, breaking up the lumps (generally in a pug mill); third, grinding of the clay, usually in a dry pan (see Section 221); fourth, tempering the clay, either in a separate pug mill or in the machine, and fifth, passing the clay through the machine and cutting off the bricks.

There are two primary types of stiff mud brick machines, viz.: The auger and plunger types. Of these the auger machines are the most numerous and generally considered the most satisfactory. The auger machine consists of a closed tube of cylindrical or conical shape, in which, on the line of the axis of the tube, revolves a shaft, to which is attached the auger and auger knives. The knives are so arranged as to cut and pug the clay and force it forward into the auger. The function of the auger is to compress and shape the clay and force it through the die. When the clay passes through the die it is compressed to as great an extent as it can be in its semi-plastic condition. The opening in the die is made the size either of the end or side of a brick, and a continuous bar of clay is constantly forced through it on to a long table. Various automatic arrangements are provided for cutting up this bar into pieces the size of a brick. If the section of the bar is the same size as the end of a brick the bricks are end cut; if the section of the bar is that of the side of a brick the bricks are side cut. With the end-cut brick the clay may issue from the machine in one, two, three or even four streams.

From the cut-off table the green bricks pass to the off-bearing belt, from which they are taken to the represses or dryers.

In the plunger brick machine the clay is forced into a closed box or pressing chamber, in which a piston or plunger reciprocates and forces the clay through the die. The action of this type of machine must of necessity be intermittent. When the plunger machine is used the clay is generally tempered in a pug mill before passing to the machine.